1. Field of the Invention
The invention relates to processes and apparatuses for perforating smooth, closed surfaces of open-cell plastic foam sheets. The plastic foam sheets with open-cell core are thermoformed into products, for example, such as trays or containers.
2. Description of the Related Art
For the production of packaging material, such as for example packaging trays, open-cell plastic foam sheets are used, which in contrast to plastic foam sheets with closed cells, are capable of absorbing liquids.
Plastic foam sheets are generally produced by a thermoplastic material being extruded together with a blowing agent, in particular a blowing gas, from an annular or slot-shaped or coextrusion die. Shortly after leaving the extrusion die, the blowing agent finely distributed in the plastic expands so that extremely fine bubbles form in the plastic foam body. Depending on process conditions with regard to the temperature and extrusion rate, bubbles of various sizes are obtained. Accordingly, the plastic foam sheet thus produced contains closely packed, closed, gas-filled cells, the gas enclosed in the cells generally being air and/or remains of the blowing agent. Such closed-cell plastic foam sheets have excellent insulating properties which are essentially attributable to the closely packed, closed, gas-filled cells.
A plastic foam sheet in which the cells are not closed but open, i.e., continuous, behaves differently than closed cells. Such a plastic foam sheet can, for example, absorb and store liquids, similar to a sponge. Such open-cell plastic foam sheets are generally produced by means of a tandem extrusion line. These plastic foam sheets are provided with a thin, closed-cell skin on the inner side and outer side. This skin is necessary in order to ensure a good thermal molding of the plastic foam sheet into a tray product. However, in order that liquid can be absorbed by the tray products produced, it is necessary to perforate the surface skin at certain points.
The plastics of the foam sheet may include polyolefins, such as polyethylene (HDPE, LPE, LLTP), chlorinated polyethylene, polypropylene (PP), polypropylene copolymers (PPC), ethylene-vinyl acetate copolymers (EVA), halogenated polyolefins; styropolymers, such as polystyrene (PS), polystyrene copolymers (PSC), acrylonitrile-butadiene-styrene copolymers (ABS), styrene-maleic anhydride, toughened polystyrenes (SPS); polyesters, such as polyethylene terephthalate (PET), polyethylene-glycol terephthalate (PETP), polybutylene terephthalate (PPT); from ethylene-propylene polymer, polyvinylchloride (PVC) and/or polyphenylene oxide.
Furthermore, the plastic may include copolymers or blends of thermoplastic polymers, of polymers, copolymers or blends of thermoplastic polymers and/or copolymers.